The present invention relates to leads for cardiac pacing and more particularly to such a lead having high fatigue resistance and tolerance for physical abuse.
Cardiac pacing by means of an implantable electronic stimulator has come into increasing acceptance for treatment of a variety of cardiac ailments and arrhythmias. Various types of pacing systems are presently available which may stimulate the patient's ventricle, atrium, or both. Various pacing modes are employed, including those commonly known as fixed rate, standby, and synchronous pacing. Except for the seldom used fixed rate mode, most of these forms of pacing are responsive to electrical signals occurring spontaneously at the heart as a result of natural cardiac activity.
In virtually all popular pacing systems, the electronic pulse generator circuit, together with its associated batteries, is positioned at a site somewhat remote from the heart itself and electrical connections to the heart are made by means of flexible leads. In the case of epicardial lead placement, i.e., where the electrode is placed on the exterior of the heart, the electronic pacer itself is typically located in the abdominal cavity. In endocardial pacing, the pacemaker circuit is often located in the subcutaneous pocket near the patient's shoulder and a lead is introduced into the patient's heart through a vein. In each case, the lead is subject to continuous flexing due to the beating of the heart and to the patient's other natural movements, including breathing. Accordingly, substantial efforts have been directed at developing leads which ade resonably pliant and highly fatigue resistant. Despite such efforts, however, lead failure remains a significant problem with pacing system longevity, increasingly so as longer-lived and more reliable battery systems are developed.
One of the more popular types of lead is one in which the body of the lead, i.e., the portion extending between the stimulation electrode and the pacer connection, comprises a helical coil of Elgiloy wire fitting loosely within an insulating tube or sheath of silicone rubber. Elgiloy is an alloy developed for its high fatigue resistance and the helical winding arrangement minimizes stress concentrations under most circumstances. Such a lead is relatively compliant and long-lived, but longer life still is desired. In installing an endocardial lead of this general construction, the lead is typically stiffened during installation by the insertion of a stylet which passes through the center of the helix, the connector at the proximal end of the pacer lead being tubular for admitting the stylet. The stylet stiffens the lead so that it can be threaded through the vein into the patient's heart and appropriately positioned within the patient's heart. The stylet is then removed before the lead is connected to the stimulation pulse generator circuit. This method of lead introduction is a desirable feature of any endocardial lead. Even with this configuration, however, life expectancy of the lead is not unlimited, and replacement may ultimately be required.
Among the several objects of the present invention may be noted the provision of a lead for cardiac pacing which provides exceptionally long life and fatigue resistance; the provision of such a lead which is highly reliable and which is resistant to physical abuse; the provision of such a lead which is easy to introduce; the provision of such a lead providing desirable electrical conduction properties; the provision of such a lead which is relatively well-tolerated by the patient's body upon implantation; the provision of such a lead which facilitates contact with cardiac tissue; and the provision of such a lead which is of simple and relatively inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.